I'll start with mine.
Twice - all members agreed to speak to each other informally (banmal). However speaking casually doesn't always mean dropping "unnie" honorific as well, which many people wrongly assume that it does. The "unnie" situation is much more complicated. I'll try to explain:
Nayeon > (Jeongyeon) = Momo = Sana = Jihyo = (Mina) > Dahyun > Chaeyoung > Tzuyu.
So this is an "unnie" chart. I think it is mostly clear, so let me get to the complicated part. The Japanese members obviously refer to each other as they'd do in Japan. Age related honorifics exist in Japanese as well, but they aren't year strict, so Momo(1996) and Sana(1996) are considered to be peers with Mina(1997), therefore Mina never calls either of them "unnie". Additionally Jeongyeon and Jihyo got added to the Japanese system in a really weird way, in which Jihyo dropped the "unnie" with all 1996 liners, but Mina continue to call Jeongyeon "unnie", hence I've highlighted them with brackets. There is a very interesting story behind this as well. Basically Jeongyeon wants Mina to abandon the "unnie" however Mina finds it weird, since she's used to it and she also wants to have more unnies.
Bonus information: they seem to speak casually with other JYP artists as well, since in one of Vlives Sana and Momo called Bang Chan and were teasing him when he started to talk formally.
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Gidle - the way they speak is very mixed so far, they use both formal and informal language, though I'd say they are mostly on a casual side. Just like with Twice, there is a foreign influence as well. Shuhua and Yuqi who are both Chinese, refer to each other simply by name and obviously talk informally. The only time Shuhua calls Yuqi unnie is for teasing purposes. Additionally Shuhua has a nickname for Soojin (Jin Jin) who is 2 years older than her, Shuhua quite literally doesn't give a fuck about K honorifics. Generally Shuhua and Yuqi seems to be constantly testing waters with other members and everyone but Minnie seem to be fine with it. Yuqi once said that Minnie gets angry if she doesn't use honorifics and that's the only known case of someone in Gidle requiring honorifics to be used. And interestingly Minnie isn't Korean but Thai, so I can only guess that Thai language is also age restrictive (someone help me out)
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Dreamcatcher: they probably are one of the strictest groups when it comes to honorifics, members would call out each other for even a minor hierarchical offense, which I'll admit, does make me stan less than I stan the other two groups. I kinda like the idea of my favorite groups being very close to each other and not having any barriers.
From what we know Yoohyeon wants to be casual with all members but not all of them agreed, specifically Sua. During one of the shows, Yoohyeon said she was upset that Sua doesn't allow them to use informal language with one another, even though other members want to. Sua then said that though they are close, they are business partners and wants to keep some boundaries between them.
But I am not sure about "other members want to" part either, because numerous times we've witnessed how other members would call each other out for not using honorifics. Specifically in one of the Weekly Idol appearances Dami had to choose a member "who sometimes crosses the line" and she chose Gahyeon for not always using formal speech.
So this is pretty much all, help me fill the holes, if any, for the above groups and try to write for the other ones.
